[C] Avoid exposing internal details in aka types

[C] Avoid exposing internal details in aka types

The current aka diagnostics can sometimes leak internal details that
seem more likely to be distracting than useful.  E.g. on aarch64:

  void f (va_list *va) { *va = 1; }

gives:

  incompatible types when assigning to type ‘va_list’ {aka ‘__va_list’} from type ‘int’

where __va_list isn't something the user is expected to know about.
A similar thing happens for C++ on the arm_neon.h-based:

  float x;
  int8x8_t y = x;

which gives:

  cannot convert ‘float’ to ‘int8x8_t’ {aka ‘__Int8x8_t’} in initialization

This is accurate -- and __Int8x8_t is defined by the AArch64 PCS --
but it's not going to be meaningful to most users.

This patch stops the aka code looking through typedefs if all of
the following are true:

(1) the typedef is built into the compiler or comes from a system header

(2) the target of the typedef is anonymous or has a name in the
    implementation namespace

(3) the target type is a tag type or vector type, which have in common that:
    (a) we print their type names if they have one
    (b) what we print for anonymous types isn't all that useful
        ("struct <anonymous>" etc. for tag types, pseudo-C "__vector(N) T"
        for vector types)

The patch does this by recursively looking for the aka type, like the
C++ frontend already does.  This in turn makes "aka" work for distinct type
copies like __Int8x8_t on aarch64, fixing the ??? in aarch64/diag_aka_1.c.

2019-10-14  Richard Sandiford  <richard.sandiford@arm.com>

gcc/c-family/
	* c-common.h (user_facing_original_type_p): Declare.
	* c-common.c: Include c-spellcheck.h.
	(user_facing_original_type_p): New function.

gcc/c/
	* c-objc-common.c (useful_aka_type_p): Replace with...
	(get_aka_type): ...this new function.  Given the original type,
	decide which aka type to print (if any).  Only look through typedefs
	if user_facing_original_type_p.
	(print_type): Update accordingly.

gcc/testsuite/
	* gcc.dg/diag-aka-5.h: New test.
	* gcc.dg/diag-aka-5a.c: Likewise.
	* gcc.dg/diag-aka-5b.c: Likewise.
	* gcc.target/aarch64/diag_aka_1.c (f): Expect an aka to be printed
	for myvec.

From-SVN: r276951